1. Field of the Invention
The present invention relates generally to a speech recognition system for an automotive vehicle, and more particularly to a speech recognition system by which a driver's spoken instructions can be reliably recognized even as engine noise fluctuates within the passenger compartment.
2. Description of the Prior Art
There is a well-known speech recognizer which can activate various actuators in response to human spoken instructions. When this speech recognizer is mounted on a vehicle, the headlight, for instance, can be turned on or off in response to spoken instructions such as "Headlight on" or "Headlight off". Such a speech recognizer usually can recognize various spoken instructions in order to control various actuators; however, there are some problems involved in applying this system to an automotive vehicle.
Usually, the speech recognizer is used in a relatively quiet environment; however, the speech recognition system for an automotive vehicle is usually used within a relatively noisy passenger compartment subject large variations in noise level. Therefore, one of the major problems is how to cope with erroneous spoken phrase recognitions caused by fluctuating background, particularly engine, noise within the passenger compartment.
In order to distinguish a spoken instruction from noise, a voice detector is usually provided in the speech recognizer. The start and the end of a spoken instruction are respectively determined by detecting whether the magnitude of a spoken instruction signal exceeds and drops below a predetermined fixed reference threshold voltage level for first and second predetermined time periods. A high noise level causes the fixed reference threshold voltage level to be exceeded for a long time. Hence, the voice detector is likely to erroneously consider this state to represent the beginning of a spoken instruction. In other words, the prior-art speech recognizer is prone to erroneous recognition due to intense noise within the passenger compartment.
A more detailed description of a typical prior-art speech recognizer will be made with reference to the attached drawings in conjunction with the present invention under DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS.